Women typically desire certain things from relationships: love, affection, security. Men's desires overlap to a degree in that we want love and affection as well, but we also want (and to some extent need), sex.
When a woman meets a man who interests and excites her, and he may interest her because of his maturity, confidence and success, she will feel an overwhelming arousal for him. She may, as happened to me, be saving herself for marriage, and toss that concept away just to have sex with him. She doesn't think 'I'm going to fuck him to use him', instead she just has an overwhelming desire to kiss him and hug him and to go all the way with him.
And we men respond to this affection; we pick up on her flirting, we recognise her desire for us. We have sex together, we share pussy and cock, and we fall in love.
We men now have what we want: love, affection and sex. The woman has what she wants: love, affection and security from a man who loves her. Subconciously she's traded sex, but if we men think with our heads as much as between our legs, we won't get involved long-term with someone who isn't truly suited to us. But if she trades sex to win our affection, and if we're soulmates and truly suited to one-another, then the end result may be years and decades of marital bliss. Bliss that includes: love, affection, respect, security and lots of sex. Indeed, some married men have sex lives that are almost beyond belief.